The present invention relates generally to gas turbine engines, and, more specifically, to noise attenuating acoustic liners therein.
A turbofan gas turbine engine powering an aircraft in flight produces noise in the inlet and exhaust ducts thereof. Inlet air flows firstly through a row of spinning fan blades which interact therewith for producing propulsion thrust with accompanying noise. A portion of the inlet air is compressed inside the engine and mixed with fuel and ignited for generating hot combustion gases which flow downstream through several rows of spinning turbine blades which extract energy therefrom, with the exhausted gas providing additional thrust and additional noise.
In order to attenuate both inlet and exhaust noise in the engine, the corresponding ducts typically include acoustic liners. A common acoustic liner is known as a two degree of freedom (2DOF) acoustic liner having two honeycomb cores joined together at an intermediate porous septum. The liner includes a face sheet over which the air or exhaust gases flow, and an opposite impervious back sheet suitably mounted in the corresponding flow duct.
The cells of the honeycomb cores are typically aligned with each other and are specifically sized for effecting Helmholtz resonators for attenuating noise. The septum is porous for acoustically coupling the outer and inner honeycomb cells for increasing the noise attenuation frequency range.
The multiple components of the acoustic liner increase the complexity of the manufacture thereof and attendant cost. In one manufacturing process, the honeycomb cores and septum are prefabricated and bonded together in an assembly. Bonding is effected by applying an adhesive along the edges of the honeycomb cells, assembling the cores on opposite sides of the septum, and curing the adhesive to bond together the cores and septum. However, the apertures of the septum are relatively small, on the order of several mils, and excess adhesive at the cell edges plugs the septum therealong and decreases the efficiency of noise attenuation.
Adhesive blocking of the septum apertures during manufacture may be decreased by more precisely controlling the manufacturing process, and providing additional steps. However, this substantially increases the cost of manufacture of the liner and is thusly undesirable.
Accordingly, it is desired to provide an improved method of manufacturing acoustic liners with increased simplicity and without adhesive blocking of septum apertures.